1. Technical Field
The present invention relates to a screw extruder for the continuous extrusion of materials with high viscosity, in particular metals such as aluminum and its alloys, the extruder including an Archimedes screw provided in a screw housing with an inlet for the feeding of the material to be extruded, a compacting or extrusion chamber and an extrusion die assembly with a die which forms the shape of the desired extruded product.
2. Description of the Related Art
Extrusion of materials with high viscosity such as aluminium requires a considerably high pressure to force the aluminum through the die block and the die, typically 100 to 500 MPa. The state of the art as regards aluminium extrusion is today and has been for the last 100 years, dominated by ram extrusion. Ram extrusion is a batch process in which a billet is loaded into a container and forced (pressed) through the die by means of a moving piston. One mentionable method for continuous extrusion of aluminum is the so-called CONFORM® process which is based on the use of a single revolving wheel as the driving force in an extrusion process and which enables the manufacture of products of unlimited length. In its simplest form, the wheel has a single groove in its periphery which accepts the feedstock and transfers the material to the extrusion zone and die.
Another continuous screw extrusion process is used in the production of lead and lead alloy profiles which is based on the Robertson Hansson extruder (U.S. Pat. No. 3,693,394). In this process the lead is fed to the extruder in liquid state and solidifies during the extrusion process.
Lead behaves differently from aluminum since it has “sliding” friction, i.e. the friction between the lead and the container material (steel) is proportional to the pressure. Aluminum and most other metals have, however, sticking friction at the extrusion temperatures, i.e. the aluminum welds to the container and the screw material.
As a result of this behavior, screw extrusion of aluminum and other sticky metals with high viscosity have been difficult and non-practical due to the enormous forces required to overcome the frictional forces between e.g. aluminum and steel.